Oh yeah. It’s going to be super difficult to guess who I am when I log into example.com and it redirects me to example.com/profile/igorpartola. Nothing to see here. Nothing at all.
However, they can set up specific experiments, and will send the results. Thus, they could set up an experiment "did users first visit example.com and later example.com/profile/igorpartola, and then have only you report that to be the case.
That example is of course not that likely and thus troublesome. What worries me, though, is the described experiment "what search terms have led to users switching search engine", and then reporting the search engine. They acknowledge this problem, but the response is "at least that's better than gathering that data server-side". True, but it's still too much data, I think.
Considering that all this tracking has to be opt-in under German law (I talked with the Landesdatenschutzbeauftragte of my state about your "experiment" here already, and they confirmed it), I’d say you can expect quite some trouble with the way you’re running this shit.
And, in contrast to Mozilla, the Landesdatenschutzbeauftragte actually uses PGP email.
It's also one way to do away with mozilla.
This is why I previously continued using Firefox and supporting Mozilla even while they put Google Analytics in their browsers’ addon menu.
But after this, I just can’t. A world without mozilla is now better than one with.
But even if it is not, hopefully the resulting media presence is enough to either kill Mozilla, or to ensure everyone at Mozilla that considers this a good idea is fired.
And yes, at the end of the day, it's up to the Datenschutzbeauftragte and the courts to decide, but I really do think that you delude yourself, if you believe this will go anywhere. They'd have locked up the entirety of Google and Microsoft multiple times already.
They have sued Google and Microsoft into the ground multiple times already. There’s a reason half of Google Photos and Facebook’s features aren’t available in Germany (including no face recognition).
Thilo Weichert, previous Landesdatenschutzbeauftragter of SH was constantly in the media due to constantly suing (and winning) against Google, Facebook and co.
Either way it is ethically questionable.
Another example of how evil mozilla has become.